CH. XIV] LIGHT AND ENERGY LOSSES IN PROJECTION 603 



The glass of which lenses are made is not perfectly transparent 

 but absorbs some light. This is especially true of the thick con- 

 denser lenses which usually look green when laid on a piece of white 

 paper. Such green lenses will be found to absorb an appreciable 

 amount of light. Some condenser lenses made of cheap glass will 

 turn purple after being in use for some time. 



843. Special light losses. The use of polarized light neces- 

 sarily entails the loss of one-half of the light in the polarizing nicol. 

 The analyzing nicol may transmit most of the remaining light but 

 generally it is turned to transmit but a small portion of it ( 884). 



In moving picture projection the shutter covering the lens while 

 the film is in motion removes part of the light. In this case it has 

 been found by careful experiment that removing all of the light 

 part of the time has exactly the same effect as removing part of the 

 light all of the time. Some shutters remove but /^ of the light 

 while others remove ~%. of the light ( 591). The latter are, how- 

 ever, sometimes to be preferred, the avoidance of flicker being of 

 more importance than the slight dimming of the image. 



ENERGY LOSSES 



844. Of the energy which is radiated by the source only a 

 comparatively small part, from 2 to 10 per cent, is of those wave 

 lengths which affect the eye, the major part of the energy being in 

 the infra-red part of the spectrum (fig. 307). This infra-red 

 radiation accompanies the light radiation and is bent by a lens in 

 very nearly the same manner. It has been found that the differ- 

 ence in focus between the infra-red and the red, for glass, is no 

 greater than the difference in focus between the red and the blue. 

 This is due to the special dispersive qualities of glass. 



845. Disadvantage of the infra-red. As the infra-red 

 radiation has such great energy and consequently so great a heating 

 effect wherever it is absorbed (fig. 307), and as at the same time 

 it has no effect upon the eye, it is advantageous to remove it as far 

 as possible. Energy losses, in so far as they are not accompanied 

 by light losses, are of advantage. 



